Implements vs. Extends When Defining a Class

Transcription

1 Implements vs. Extends When Defining a Class implements: Keyword followed by the name of an INTERFACE Interfaces only have method PROTOTYPES You CANNOT create on object of an interface type extends: Keyword followed by the name of a BASE CLASS Base class contains method IMPLEMENTATIONS Allows INHERITANCE!! You CAN create objects of that base class type 0

2 Example: People at University Base class: person Derived classes: student, faculty, administrator Derived from those: undergrad, grad, instructor, professor, Person Student Faculty Administrator Undergrad GradStudent Instructor Professor 1

3 University Person Example extends Person class: Student instance variables: int admityear double gpa methods: Student( ) [various] int getadmityear( ) double getgpa( ) void setadmityear( int ) void setgpa( double ) String tostring( ) boolean equals( Student ) class: Person instance variables: String name String idnum methods: Person( ) [various] String getname( ) String getidnum( ) void setname( String ) void setidnum( String ) String tostring( ) boolean equals( Person ) extends Person class: Faculty instance variables: int hireyear methods: Faculty( ) [various] int hireyear( ) void sethireyear( int ) String tostring( ) boolean equals( Faculty ) 2

4 Person class Lets inspect the (simple) code. 3

5 Student class? Memory diagram? this vs. super Consider: Person q = new Student(); 4

6 Implementing a subclass Lets implement the Student class Constructors: Typical (with parameters) No arg Constructor Copy Constructor Overridden methods: tostring equals 5

7 Overriding vs. Overloading Overriding: a derived class defines a method with same name, parameters as base class Overloading: two or more methods have the same name, but different parameters Example public class Person { Base class setname( ) public void setname( String n ) { name = n; public class Faculty extends Person { public void setname( String n ) { Overriding super.setname( The Evil Professor + n ); public void setname( String first, String last ) { Overloading super.setname( first + + last ); 6

8 Early vs. Late Binding Consider: Person p = new Student(); System.out.println( p.tostring() ); Which version of tostring Person or Student is called? Early (static) binding p is declared to be of type Person Therefore, the Person version of tostring is used Late (dynamic) binding The object to which p refers was created as Student object Therefore, the Student version of tostring is used Java uses late binding (C++ by default uses early binding) Early binding is more runtime efficient (decisions about method versions can be made at compile time) Late binding respects encapsulation (object defines its operations when it is created) 7

9 Polymorphism Java s late binding makes it possible for a single reference variable to refer to objects of many different types. Such a variable is said to be polymorphic (meaning having many forms). Example: Create an array of various university people and print. Person[ ] list = new Person[3]; list[0] = new Person( "Col. Mustard", " " ); list[1] = new Student ( "Ms. Scarlet", " ", 1998, 3.2 ); list[2] = new Faculty ( "Prof. Plum", " ", 1981 ); for ( int i = 0; i < list.length; i++ ) System.out.println( list[i].tostring( ) ) Output: [Col. Mustard] [Ms. Scarlet] [Prof. Plum] What type is list[i]? It can be a reference to any object that is derived from Person. The appropriate tostring will be called. 8

10 Public, Protected, Package(default) and Private Select which level of visibility Access Levels Access Level/Group Class Package SubClass World public Y Y Y Y protected(avoid) Y Y Y N package (default) Y Y N N private Y N N N 9

11 Shadowing Can we override instance variables just like methods? Yes, but be careful! Overriding instance variable is called shadowing Shadowing hides instance variables of base class (can still access them using super.varname in subclass, but not in outside world ) public class Person { String name; public class Administrator extends Person { String name; // name refers to Administrator s name Confusing! Better to pick a new variable name 10

12 Example of Overloading/Overriding public class Base { public void m (int x) { public class Derived extends Base { public void m (int x) { public int m (int x) { public void m (double d) { Error! duplicate method declaration Overloading Overriding // The following appears in the same package as above Base b = new Base( ); Base d = new Derived( ); Derived e = new Derived( ); b.m (5); d.m (6); d.m (7.0); e.m (8.0); calls Base:m(int) calls Derived:m(int) Error! Since d is declared Base, the compiler looks for Base:m(double) Doesn t exist! So this does not make it past the compiler, even though Derived:m(double) is defined! calls Derived:m(double) 11

13 Object Recall: inheritance induces is-a hierarchy on classes Undergrad is-a Student Student is-a Person etc. Person is-a.? Person is-a (n) Object Student is-a (n) Object Object Person Student Faculty Administrator Undergrad GradStudent Instructor Professor 12

14 More on Object Special class at top of class inheritance hierarchy Defined in java.lang Every class is derived (either directly or indirectly) from Object e.g. public class Foo { is equivalent to public class Foo extends Object { Structure of Object No instance variables A number of methods, including: tostring() equals (Object o) Let s look at the Javadoc 13

15 Class vs. Type Information In Java Every object is in one class (the one it was created from using new) Objects may have many types (all those that class is based on) Interfaces Superclasses E.g. consider Student bob = new Student(); Person p = bob; Class of object pointed to by bob and p is Student Type of object can be Student, Person, Object, etc. 14

16 Accessing Type Information instanceof Java boolean operator (not a method) Returns true if given object is-a (n) object of given type E.g. Student carol = new Student ( ); if (carol instanceof Person) // true, because carol is-a Person 15

17 Object Casting Recall casting in primitive types Casting: conversion of elements from one type to another Widening Conversion (always OK) double x = 3; // 3 (int) widening conversion to double Narrowing Conversion Must use explicit type conversions to perform this casting int x = (int)3.0; // 3.0 explicitly cast to int Similar notions can be found with object types also Upcasting Casting a reference to a superclass (casting up the inheritance tree) Always OK Just ignore the parts that were added by the subclass Downcasting Casting a reference to a derived class Requires explicit casting operator Can cause runtime error 16

18 Object Casting Person p = new Person(); Student s = new Student(); Person tricky = new Student(); Person x = s; Student y = p; Student y = (Student)p; Student y = tricky; Student y = (Student)tricky; (Faculty)s (Faculty)tricky Downcast Does not compile Compiles, but throws exception Downcast Does not compile Works fine Does not compile Compiles, but throws Exception 17

19 Safe Downcasting Illegal downcasting results in a thrown ClassCastException at run-time Q: Can we check for the legality of a cast before trying it? A: Yes, using instanceof Example Given: ArrayList of university people Want: Print the GPAs of the students Solution approach Iterate through list Print GPAs only of Students 18

20 equals() Reconsidered Recall definition of equals() in Person public boolean equals (Person p) { if (p == null){ return false; return name.equals(p.getname()) && idnum.equals(p.getidnum()); in Student public boolean equals( Student s ) { if (s == null){ return false; return super.equals(s) && admityear == s.admityear && gpa == s.gpa; What does following do? public static void main (String[] args) { Student bob = new Student ("R. Goode", " ", 1998, 3.89); Faculty bob2 = new Faculty ("R. Goode", " ", 2005); System.out.println (bob.equals (bob2)); true is printed! 19

21 A Better equals() Take Object as parameter Check for non-null-ness of parameter Check that class type is correct Then do other checks For example in Person: public boolean equals (Object o) { if (! (o instanceof Person)) { return false; Person p = (Person)o; return name.equals(p.getname()) && idnum.equals(p.getidnum()); Similar improvements can be made to Student, Faculty Now bob.equals(bob2) returns false 20

22 Multiple Inheritance? Intuitively useful to be able to inherit from multiple classes (multiple inheritance) Person Student Athlete Faculty StudentAthlete But Java does not allow this 21

23 Why Does Java Disallow Multiple Inheritance? Semantic difficulties! Consider StudentAthlete Objects would get name field from Student Objects would also get name field from Athlete Duplicate fields: what to do? Some languages (e.g. C++) do allow multiple inheritance Person Student Athlete StudentAthlete 22

24 Can We Achieve Some of Benefits of Multiple Inheritance in Java? Yes, using interfaces + inheritance Idea: use inheritance for one of inherited classes, interfaces for others Interfaces ensure that relevant methods are implemented Example public class Person { public class Student extends Person { public interface Athlete { public String getsport (); public void setsport (String sport); public class StudentAthlete extends Student implements Athlete { Objects of type StudentAthlete are Students They also can be wherever objects matching Athlete are required 23

25 Interfaces and Constants Interfaces can also contain public final static variables Sometimes interfaces are used to provide consistent definitions for constants throughout an application Example public interface Months { public final static int JANUARY = 1; public final static int FEBRUARY = 2; public final static int MARCH = 3; public final static int DECEMBER = 12; public class MonthDemo implements Months { public static void main( String[ ] args ) { System.out.println( "March is month number " + MARCH ); Because MonthDemo implements Months, it has access to the constants 24